The utility of many laboratory procedures requires the precise measurement of a small volume of liquid. It is not uncommon to require the accurate dispensing of volumes of less than 10 milliliters. Even for larger volumes, it is desirable that the accuracy of the measurement be assured and that it be repeatable over many pipetting operations. The design of manual pipettes generally assures that a precise predetermined volume of liquid is aspirated into the pipette reservoir, but the discharge of that precise volume is not always achieved. Quite often, a small quantity of the liquid remains after the discharge stroke of the piston is completed. And unless all of the liquid is discharged from the reservoir, the precision built into the pipette mechanism is negated. The liquid remaining may be a droplet formed at the orifice of the reservoir which is not discharged with the rest of the liquid, or it may be a thin film of liquid adhering to the wall of the reservoir. This often results because the liquid to be expelled is not driven out of the reservoir directly by the pipette piston but by a column of air located between the liquid and the piston. The column of air generally will compress and then expand to drive the liquid out of the reservoir depending on the velocity that the technician imparts to the piston during the discharge stroke (which velocity, of course, can vary from stroke to stroke), the density of the liquid being pipetted, and its viscosity. The latter two factors affect the force necessary to move the liquid from its position of rest in the reservoir.
It will be appreciated that the complete discharge of liquids from pipette reservoirs is important in the use of pipettes for critical laboratory procedures. This is especially true where small volumes, in the order of 10 milliliters or less, are being measured. In such cases, the volume of a droplet remaining on a pipette tip will be an appreciable part of the volume initially aspirated. So much so has this become a consideration that many pipette arrangements have been disclosed that will expell essentially all of the liquid initially drawn into the pipette. The more recent proposals have resorted to the provision of a so-called overblow piston that provides a relatively large quantity of compressed air to blow the liquid residue from the pipette reservoir. See, for example, U.S. Pat. Nos. 3,834,590; 3,933,048; and 3,935,734.
While the pipettes disclosed in the aforementioned patents function as intended, they are subject to the limitation that their discharge operating characteristics are determined by the manufacturing specification, and that these characteristics cannot be altered or adjusted by the technician using the pipette even though they should be altered to take into account the density or the viscosity of the liquid being pipetted. Since different liquids are pipetted from time to time, it would be desirable to have a pipette which embodies an adjustable discharge mechanism.